string(MAKE_C_IDENTIFIER): Add UPPER flag
This adds a new UPPER flag to the string(MAKE_C_IDENTIFIER) sub-command to enable additionally converting any lowercase letters of the resulting string to uppercase.
It's not super important to add it, but it might be helpful to simplify the common creation of C identifiers which need uppercase letters, such as the HAVE_* symbols, commonly used in C/C++ code.